Error message provides suggested fix that does not compile

[dralley]

When I attempt to compile the code:

fn main() {
    let mut sum = 0i32;
    
    for i in 0..10 {
        sum += i.pow(2);
    }
    
    println!("{}", sum);
}

I get back an error message with a 'help' suggestion:

error[E0689]: can't call method `pow` on ambiguous numeric type `{integer}`
 --> test.rs:5:18
  |
5 |         sum += i.pow(2);
  |                  ^^^
help: you must specify a type for this binding, like `i32`
  |
4 |     for i: i32 in 0..10 {
  |         ^^^^^^

error: aborting due to previous error

This is a great error message however the suggested code unfortunately does not work. Adding : i32 to the previous code sample as suggested will also fail with the error:

error: missing `in` in `for` loop
 --> test.rs:4:10
  |
4 |     for i: i32 in 0..10 {
  |          ^ help: try adding `in` here

error: expected expression, found `:`
 --> test.rs:4:10
  |
4 |     for i: i32 in 0..10 {
  |          ^ expected expression

error: aborting due to 2 previous errors

What does work, is changing 0..10 to 0i32..10. The rustc-suggested syntax does seem reasonable to me (as a newbie) so it's a little surprising that it's invalid.

rustc version:

rustc 1.27.0 (3eda71b00 2018-06-19)
binary: rustc
commit-hash: 3eda71b00ad48d7bf4eef4c443e7f611fd061418
commit-date: 2018-06-19
host: x86_64-unknown-linux-gnu
release: 1.27.0
LLVM version: 6.0

[zackmdavis]

Duplicate of #51634; messaging improved in #51670. Nightly output:

error[E0689]: can't call method `pow` on ambiguous numeric type `{integer}`
 --> 51972.rs:5:18
  |
4 |     for i in 0..10 {
  |         - you must specify a type for this binding, like `i32`
5 |         sum += i.pow(2);
  |                  ^^^

error: aborting due to previous error

(Although this doesn't emit the bogus suggestion, this could arguably still be improved; the actual fix is to specify the type on the range (0_i32..10), not the i.)

[dralley]

@zackmdavis Thanks. Do you know if there is a reason why, theoretically, rust could not support this syntax eventually?

[zackmdavis]

@dralley There's an unstable type_ascription feature that makes : Type notation work in a lot more places (although for-loop bindings don't seem to be one of them), but the tracking-issue discussion makes it looks like folks don't want to stabilize it in its current form.